The option was fine, but now that there is a dedicated Alpine Linux
task, which uses BusyBox, it seems redundant.
(See: ci/test/00_setup_env_native_alpine_musl.sh)
So remove the USE_BUSY_BOX option, along with the BINS_SCRATCH_DIR env
var.
Also, enable pipefail in the ci/test/00_setup_env.sh script, while
touching it.